1. Field of the Invention
The present invention relates to a process for producing a sintered body of a ceramic composition for voltage non-linear resistor which comprises a main component of ZnO and a minor component of a transition metal of Co, Mn, Ni or an alkaline earth component or a rare earth component etc. More particularly, it relates to a process for producing a sintered body of a ceramic composition for voltage non-linear resistor which has remarkably large voltage non-linearity and large discharge capacity.
2. Description of the Prior Art
Recently, the uses of semiconductors and semiconductor circuits have been increased in measuring instruments, control instruments, communication instruments and power source instruments because of remarkable development of semiconductors and semiconductor circuits such as thyristors, transistors and IC etc. Thus, the miniaturization and high function of these instruments have been attained. On the other hand, withstand voltage, surge voltage resistance and noise resistance of these instruments and parts thereof have not satisfactorily developed so as to follow the developments of these instruments. Thus, it is important to protect these instruments and parts from abnormal surges or noises or to stabilize these circuit voltages.
It has been required to develop economical substances for voltage non-linear resistors which has excellent voltage non-linearity, excellent discharge capacity and long life characteristic in order to overcome the problems.
The voltage non-linear resistors (referring to as varistors) such as SiC variators and Si diode varistors; and zener diodes have been used for said purposes. Recently, varistors comprising a main component of zinc oxide and an additive have been proposed.
The voltage-ampere characteristic of a varistor is usually shown by the equation EQU I=(V/C).sup..alpha.
wherein V designates a voltage applied to the varistor and I designates a current passed through the varistor and C designates a constant corresponding to the voltage when the current is passed.
The exponent .alpha. can be given by the equation EQU .alpha.=log.sub.10 (I.sub.2 /I.sub.1)/log.sub.10 (V.sub.2 /V.sub.1) (1)
wherein V.sub.1 and V.sub.2 respectively designate voltage under passing the current I.sub.1 or I.sub.2.
A resistor having .alpha.=1 is an ohmic resistor and the nonlinearity is superior when the .alpha.-value is higher. It is usual that .alpha.-value is desirable as high as possible. The optimum C-value is dependent upon the uses of the varistor and it is preferable to obtain a sintered body of a ceramic composition which can easily give a wide range of the C-value.
Thus, it is preferable to use the substance which easily give a wide range of values C.
The conventional silicon carbide varistors can be obtained by sintering silicon carbide powder with a ceramic binding material. The non-linearity of the silicon carbide varistors is based on voltage dependency of contact resistance between silicon carbide grains. Accordingly, the C-value of the varistor can be controlled by varying a thickness in the direction of the current passed through the varistor. However, the non-linear exponent .alpha. is relatively low as 3 to 7. Moreover, it is necessary to sinter it in a non-oxidizing atmosphere. On the other hand, the non-linearity of the silicon varistor is dependent upon the p-n junction of silicon whereby it is impossible to control the C-value in a wide range.
The non-linearity of the zener diode varistor is also depending upon the p-n junction of silicon. Thus, the voltage non-linearity is remarkably great, however it is difficult to prepare the element for high voltage. Moreover, the discharge capacity is small and the endurance to surge is weak to be disadvantages.
Recently, varistors comprising a main component of zinc oxides and a minor component of transition metal such as cobalt manganese and nickel have been developed and practically used.
The non-linearity of the latter varistor is caused by the sintered body itself to be greater than 30 of the index .alpha.. However, the discharge capacity is not always satisfactory for the purpose of the protection of the circuit or elements and the field of the uses of such varistors is limited.
It has been proposed to improve the discharge capacity by coating a glassy material on a sintered product, heat-treating it at high temperature to diffuse it.
However, this method has the following disadvantages.
(1) The steps are increased to increase the cost;
(2) The mass production is not easily carried out because the elements are easily adhered each other by the grassy material after coating the glassy material; and
(3) The diffusion of the glass is highly affected by the distribution of the temperature in the oven whereby it is difficult to produce products having uniform characteristic at high non-defect ratio.
Thus, the cost of the element is remarkably high and the elements having high discharge capacity have not been commercialized, disadvantageously.